The influence of adjacent tissue on hypocotyl hook opening

The influence of the cotyledons and the tissue forming the proximal end of the hook in Helianthus annuus L. and Phaseolus mungo L. seedlings growing in the dark has been examined by comparing the growth of whole and decapitated seedlings in which the hook region was demarcated by the use of resin beads as surface markers. It is shown that although decapitated seedlings straighten and whole seedlings maintain a hooked configuration, the curved region in the control seedlings corresponding to that remaining in the decapitated plants also undergoes straightening in the course of the experiment. Maintenance of the hook in controls is thus a consequence of the continuous shifting of the differential growth to a more apical region which decapitation removes. The significance of this finding in relation to the suggestion that cotyledons export an endogenous hook-shutting factor is indicated.